In conventional design schemes of semiconductor analog integrated circuits, a circuit simulation tool known as "SPICE" (Simulation Program with Integrated Circuit Emphasis) is widely used. For electronic circuits constructed by combining active and passive elements, "SPICE" calculates the circuit equation by connection information and model variables which describe the operation characteristic of each component which has been given in advance, and simulates the operation of the electronic circuit by solving the circuit equation.
Though "SPICE" performs the circuit simulation and the analysis of the operation of the electronic circuit properly from entered circuit information and model parameters of circuit element, it does not have the best intelligent function like that it analyzes the entered circuit information a high degree, goes the malfunction and point that and developing more, add the improve change automatically to design electronic circuits as evade the malfunction.
In the design scheme of analog integrated circuits, there are various ways and circuits for achieving the same function. Accordingly it is difficult to determine which way or circuit is the best or optimum circuit design scheme for a given specification by a certain regulation. In other words, it requires a professional skill fostered by a long-term experience to design a good analog integrated circuit.
Such situations in analog integrated circuit design schemes have hindered progress of design tools with intelligent circuit analysis and automatic design functions which have keen highly demanded for many years. Further, such situations have made development of automatic circuit design scheme, i.e., CAD scheme (computer aided design scheme of circuits) of the analog integrated circuits behind that of digital integration circuits.
However, there are proper and improper combinations of elements to the specification given to analog circuits. Thus, it is partially possible to determine good or bad designed circuits under certain preconditions. Accordingly it is possible to improve CAD tools to advance the efficiency of circuit design schemes, not to be a fully automatic design scheme by Using such design rule.
in analog integrated circuits, a "feedback" scheme is often used to make circuits stable, highly accurate and to lower their impedances. However this feedback scheme brings a difficult problem of "oscillation" if the scheme carried out under a wrong setup. When a circuit loop forming a feedback path is made in a positive feedback loop, the circuit loop frequently becomes unstable, except in cases using Schmitt circuits, bootstrap circuits and negative impedance circuits. Though the circuit loop forming the feedback path is made in a negative feedback loop for a stabilization, it changes many times un-intentionally to a positive feedback loop by a phase inversion in a high frequency band, so as to cause a high frequency oscillation.
Accordingly, when a designed circuit has a feedback loop, it is important to determine whether the feedback loop is a positive feedback loop caused by incorrect wiring, as well as to determine that, if the feedback loop is the negative feedback loop, it has an oscillation condition in a high frequency band.
As such a fault determination scheme for a feedback loop, there is a determination scheme based on an open loop characteristic (gain-phase characteristic). One example of the scheme will be explained by the circuit, as shown in FIG. 1. The circuit shown in FIG. 1 is a voltage-follower circuit based on an operational amplifier which comprises a non-inverted input terminal Ia, an inverted input terminal Ib, and an output terminal Oa directly coupled to the inverted input terminal Ib. This voltage-follower circuit is often used as an impedance-changing circuit in analog integrated circuits, especially in analog integrated circuits for audio signal circuits and bias circuits.
This voltage-follower circuit operates as a 100% negative feedback loop for direct currents. However, in a high frequency band the voltage-follower circuit has a possibility of causing oscillations, because the phase in the circuit greatly shifts due to parasitic capacitance, etc., in transistors coupled to a node 4. The oscillation condition presents in a case that, in the feedback loop starting from the output terminal Oa and returning to the inverted input terminal Ib of the operational amplifier as shown in FIG. 1, the gain in a cycle of loop exceeds "1" for the frequency in which a phase in a cycle of loop shifts by 180.degree.. Therefore, as an analysis for determining oscillations, it is enough to calculate the gain-phase characteristic of the feedback loop by inserting a dummy signal source for the analysis in any position of the feedback loop. Though there are various schemes as the practical method of this determining analysis, the article "The Design Scheme of the Transistor Circuit by SPICE", (CQ Publish Company, written by Okamura, first published Jun. 10, 1992, pages 73-74) is known as a scheme to obtain a correct result rather easily by calculating the open loop characteristic with its loop closed.
Accordingly, as shown is FIG. 2 as to the circuit in FIG. 1, the risk of oscillation can be determined from both the current gain of the round loop and the frequency characteristic of the phase obtained by inserting the current signal source Ig and current sensor Vx, Vy, and practicing the AC-analysis by SPICE, and finding the ratio I(Vx)/I(Vy) of the currents I(Vx) and I(Vy) flowing through the current sensors Vx and Vy. For example, in the case that the gain-phase characteristics shown in FIGS. 3(a) and 3(b) is obtained by this analysis, oscillation is found at the frequency around the frequency fa because the gain exceeds OdB (once) at the frequency fa that phase turns 180.degree.. To the contrary, in the case that the gain-phase characteristic shown in FIGS. 4(a) and 4(b) is obtained by this analysis, oscillation will probably not occurred under this condition because the gain is under OdB (once) at the frequency fa that phase turns 180.degree..
The risk of oscillation can also be determined from both the current gain of the round loop and the frequency characteristic of the phase obtained by cutting the same position and inserting a voltage signal source Vg as shown in FIG. 5, in place of the current signal source Ig in FIG. 2, and practicing the AC-analysis, and finding the ratio V2/V3 of amplitude voltages V2 and V3 at the input and output ends of the voltage signal source Vg. This method of the determining analysis is the same case as shown in FIGS. 3(a), 3(b), 4(a) and 4(b).
However, the signal source for use in the determining method as described above is not allowed to be inserted anywhere in the loop. In FIG. 1, the feedback path is formed by joining the output terminal Oa and the inverted input terminal Ib in the operational amplifier, but when the feedback path is formed, the loop is formed by two loops, i.e., the loop circulating in an order of the nodes; 3.fwdarw.8.fwdarw.3 and the loop circulating in an order of the nodes; 3.fwdarw.4.fwdarw.8.fwdarw.3, as shown in FIG. 1. And a determination whether this loop actually oscillation or not is decided by the interaction of this double loop.
Therefore to determine the oscillation of a feedback loop, the accurate determining analysis cannot be done without inserting the signal source into the signal paths sharing these two loops. In the circuit of FIG. 1 a proper position to insert the signal source into the loop can be selected from the positions A, B and C, while the positions D, E, F, G are not proper for this purpose. However, in fact, the intuition of designer was chiefly depended upon because there were not a clear way to determine such a suitable position.
In the case of a simple circuit as shown in FIG. 1, it is rather easy for an experienced circuit designer to determine that the positions A, B, C, are suitable for inserting the signal sources. However, as to actual LSI, especially large scale LSI and LSI conducting complex signals, as feedback loops become very complicated or loops exist of very wide range of circuit elements, it is difficult to seek the feedback loop itself. If it is found, it is very difficult to determine an accurate position for inserting the signal source. At the stage of designing circuits, there are many examples where the signal source inserted into a wrong position and analysis of oscillation is practiced with the incorrect result that there is no risk of oscillation being obtained. Then, when a sample is produced as an experiment, unexpected oscillation occurs. This is one reason why only expert person can design a good analog integrated circuit, and one of the big problems in designs of analog integrated circuits.
As a result of using the above-mentioned scheme, in the case judging that it is clear to oscillate or there is a risk of oscillation because of product dispersion, generally the scheme avoids oscillation by setting a built-in capacitor with a small capacitance in order of pF (pico-Farad) for the phase compensation between the node 8 and the node 5 or between the node 8 and GND in the circuit of FIG. 1. In this case, the smaller the phase compensation is, the more it is economic, so one first applies a capacitor having a small capacitance and performs the determining analysis of oscillations, as described above.
And on observing results of the determining analysis, as shown in FIGS. 3(a), 3(b), 4(a) and 4(b), the capacitance value of the phase compensation capacitor in input file is incremented step-by-step and the same analysis is repeated every time. In this way, until the oscillation conditions at a prescribed preset margin are not satisfied, the scheme continues this procedure for seeking out the most suitable phase compensation capacitance. This procedure is illustrated in FIG. 6.
In the conventional design procedure as described above, it takes a long time to rewrite the phase compensation capacitor in the input file by determining a risk of oscillation by checking the calculation result with the oscillation condition, and to perform various operations on practicing the SPICE in comparison to the time for analyzing in the actual computer. In other words, as a result, a TAT (turn around time) in the design operation using a circuit simulator becomes long and the efficiency of the design operation become worse in spite of using computers.
As described above, the conventional design scheme using the circuit simulator for overcoming oscillations of feedback loops in analog integrated circuits very frequently encounters problems or difficulties as follows.
1. It is difficult to seek unintended positive feedback loops and negative feedback loops with risks of oscillation, and these tend to be overlooked.
2. It is difficult to determine an accurate inserting position of the signal source for determining the operation, so it is inserted into a wrong position, and an accurate result cannot be obtained.
3. A TAT scheme of computer operation up to obtain the most suitable phase compensation capacitance for avoiding the oscillation becomes long, and the efficiency of the design operation also becomes insufficient.
Thus, improvements for the above problems have been desired.